Work on the purification of fluorides has intensified greatly because of interest in such applications as heavy metal fluoride glass optical fibers, tunable solid state lasers, and dielectric layers for complex semiconductor structures. Initially rapid advances were made in increasing the purity of these materials, but recent progress has slowed although impurity levels remain orders of magnitude higher than can be tolerated in some applications.
For instance heavy metal fluoride glasses show potential for fabrication of extraordinarily low loss optical fibers operating in the 2-4 .mu.m region of the infrared. One of the most serious problems is contamination with certain divalent transition metals, some rare earth ions, and the hydroxyl ion. These impurities have strong absorptions in the optimum optical region. It is necessary to attain impurity levels of one part per billion for some of these contaminants to exploit the potential for these glasses.
A number of purification approaches are used at present, including wet chemical processing and vapor treatment. Wet processing is well known, and has limitations in the degree of purification because of recontamination from the background levels of contaminants present in the processing chemicals. Current vapor preparation techniques are limited to dealing with the existing fluoride compound, and suffer from low or non-existent thermodynamic driving force for removal of the contamination. The "reactive atmosphere process" (RAP) has been used to purify a number of starting materials, and has an important effect on the hydroxyl content, but rather little effect on the other contaminants. Physical vapor transport (sublimation) has been used to purify ZrF.sub.4 and AlF.sub.3 BaF.sub.2 and GdF.sub.3 have been purified by subliming the transition metals out of them, showing greater than an order of magnitude improvement in Fe.sup.2+, but little effect on the other contaminant ions. Sublimation is limited by the ratio of the vapor pressure of the contaminant species to the desired compound.
One of the basic limitations of such processing is that the starting materials are often of limited purity, and the process is required to provide more purification than reasonable. In addition, there are some fundamental limitations on the amount of contaminant than can be practically removed because of similar vapor pressure of the contaminant compound over its solid solution with the major compound.